Pickup tube beam-interrupting circuit



Sept. 23, 1952 F. J. soMERs I PICK-UP TUBE BEAM-INTERRUPTING CIRCUITFiled Feb. 18, 1947 Patented Sept. 23, 1952 PICKUP TUBEBEAM-INTERRUPTING l cntcmr Frank J. Somers, Rockville Centre, N. Y.,assignor Y to Radio Corporation of America, a corporation of Delaware IApplieation February 1s, 1947, serial No. 729,213

2 Claims. (Cl. P18-7.2)

The presen-t invention relates to television systems, and moreparticularly relates to a television .transmitting system employing astorage-type camera tube having a so-called image section. In this formof camera tube, light from an optical image is caused to fall upon aphoto-sensitive electrode in the camera tube thereby to causephotoelectric emission from the electrode surface. Such emission is inthe form of an electron image corresponding point-for-point to theoptical image.V l l The electron image thus released from the surface ofthe photo-sensitive electrode is drawn down the camera tube under theiniiuence of an accelerating eld and impinges on a mosaic electrode.This impingement of the electron image on the mosaic sets up on thelatter an image in stored electrostatic charge, the value of this.

frame or 60 fields per second. When it is desired to utilize a. systemincluding an image type camera tube of the above nature in connectionwith the televising of images recorded on motion picture film, certainfactors must be taken in-to consideration. One of these factors resultsfrom the difference between the presently standard black-and-Whitetelevision field frequency of 60 per second (30 Vframes doubleinterlaced) and the standard motion picture projection frequency of 24iilm `frames per second, and various arrangements have been devised tocorrelate these operating frequencies so that Ythe apparent movement ofthe lm subject matter is not unnaturally accelerated. One such methodresides in the use of an intermittent projector in which alternate lmframes supply two television images for two successive televisionfields, while the remaining'film frames supply three television imagesfor three television elds. One example of `such a system is disclosed inUnited States Patent No. 2,082,092, issued June l, 1937, to A. V.

4 l3edford.

` 'in a further United States Patent No. 2,303,960,

issued December l, 1942, to S. W. Seeley, there is disclosed a motionpicture projector for television usein which a regular intermittentmotion is imparted to the nlm, and a disc shutter employed which isrotated at a speed which will result-in the projection of 60 lightimages per second on the light-responsive electrode of the camera tube,the shutter opening being so dimensionedgthat the light images areactually projected on the lightresponsive electrode of the camera tubefor only tial.

the time period between each successive transmission of the separatetelevision iields-that is, between each successive scanning operation.In an arrangement of this nature, the light-responsive electrode of thecamera tube is scanned by the cathode ray scanning beam in substantiallycom'- plete darkness, this scanning operation acting to produce videooutput signals from the camera tube by removing, by neutralizing, Vorotherwise, the charge image. The next succeeding charge image is thenproduced by the projection of a film image on the light-responsiveelectrode during the time that the cathode ray scanning beam is returnedto its starting position in preparation for another field-scanningcycle.

In such operation, the use of a disc shutter,`"or other means forcutting off light from the camera tube during the held-scanningoperation, is essen- Furthermore, this shutter must be accurately speedcontrolled so as to maintain the intermittent movement of the iilm` inpre-established relationship relative to the beam deflection generatorsassociated with the camera tube.

According -to one feature of the present invention, means are providedwhereby the rotating disc shutter normally used in lm projectiontelevision systems of the nature described above may `be dispensed with.Furthermore, an overall simplication of the transmitting system isachieved, since no moving parts need be utilized tosupplement thestandard projector unit. Briefly, this result is accomplished byapplying to the photosensitive electrodeof the camera tube a potentialwhich has alternate periods of positive and negative polarity, and bycontrollingthe application of this potential so that its positiveperiods coincide in time with the periods during which the mosaicelectrode of the camera tube is being scanned by the cathode rayscanning beam. Such a positive potential on the photo-sensitiveelectrode of the camera tube is effective during the periods of itsapplication to prevent the release of photo-electrons from thephoto-sensitive electrode Ito the mosaic. It thus has the same effect`as if no light were reaching the photo-sensitive electrode. Actually,however, such a prevention of photoelectric emission during the scanningintervals, in accordance with applicants disclosure, is not dependentupon the presence, or absence, of light impinging the photocathode.

The negative periods of the potential applied to the photo-.sensitive4electrode of. the .camera tube occur respectively during .all v'o rfaportion of the retrace, or snap-back, periods of the cathode rayscanning beam, and permit photoelectric emission from this electrodeduring these nega- Hence, the charge `image .lon tfhe mosaic isestablished, o-r built up, during retrace time, and in order to preventimage distortion T ...the cathode rayscanning YLbeam v.is .cut off,org...

tive periods.

blanked, during such retrace periods.

By a proper choice and selection of the time when positive voltagepulses applied to ithe photosensitive electrode of the camera tube-With.'r.ev

spect to the time when the intermittent movement `of the motion picturenl moccurs, the film movement may be made "to occur during the `field-sscanning periods of the cathode ray beam..` Since no photoelectricemission occurs'duringfthese lscanning intervals, thejlm movementf'willnot adversely aiTect the vcharge 'images developedbn the .camera tubemosaic. ,Furthermore .the `1121/2 ratioA between nlm. movement landtelevision field yfrequency., .as .setforth in the above-'mentionedSeeley.patent, maybe retained.

,The present. .invention ymay also be utilizedin .connection withso-calle'd field .pickups 'to .eiec- .tivly .fstop? .fast-moving.objects by Vapplying to the .camera ftlibe-Ihoto-se'nsitive .electrode,a potential having very .short vnegative periods, sayin theprderof,371/1000 Second, .As afreslili?, .Dho'toelece tric .emissionyWouldocz'cur only .during .these Yrelatively short intervals, and'blurringfo'f ythe .repro- `iduced image wouldbe eliminated..'Thescanning ,periodsfin sucha .case could retain their normal durationor -could be lengthened, as desired.

' .Onembject ofthe .present invention, therefore, .is-.to provide atelevision .transmitting .systemutilizing a camera tube having. asepara-tephoto- .sensitive electrode, and '.to Vprovide afmetho'd .and

.means whereby `the.pho'toelectriic vemissionfrorn 'this .electrode may.be .conveniently controlled.

,Another object -o'fthe invention A.iste provide,

in a television .transmitting'system.of.the;.ltype in which opticalimages .are projected. .froma motion picture fllm'or other .recording.medium on tothe separate .photo-.sensitive.electrodey .of -a cameratube, means whereby the usualidiscshuttenor other means for .cuttingoifthe light from y,bedispensed with. y f

.A .still .furtherobject of the l'invention fis .to`provide-a,television transmitting system in which 'a `camera tube'maybe so controlled-as tofstop 'the :surf-ace .of the ,photo-sensitiveAelectrode., .may

fast-moving objectsgand in which -thisvfco-ntrolling means utilizes .no.movingparts.v

. .Other objects .and advantages Will `he apparent from .the .following-.description-,ofa vpreferred V.form -of .the rinvent-ion' and .from thesdrafwing, in

.ferred form -of thepresent .inventionas .applied to `la televisiontransmitting system lin which-the Vimages to .he televised .are recordedon motion ,picture.lm; and

LFfig. 2 Yis a .chart illustrating v. fa rious:timefrela-4 tions`present inthe circuit vof Figi.. Referring .first to Fig. 1, there isshown..r .supply ree'l |0 from which vthe vnlm-to beshown b y televisionis withdrawn. The 'film |2.is passed 'over 'an 'idler 'roller I4 andthen through a lm gate I6. .The nlm l2 is drawn through the gate I6 inan intermittent manner by means of a suitable form of film drivemechanismA I8 which may, for example, be of the type shown in UnitedStates Patent No. 2,251,786, granted August 5,

`J. 9$t1 ,.i o 1.3,. Epstein.. .Ille,.interrn ittent filmadyanceui'leehanism il; hence Will; not be described in detail, and itwill merely be stated that .the mechanism I8 is o-ne of any suitableform lwhich acts to pull the nlm l2 through the gate -15 at the ratev0F24 film frames per second when driven by a synchronous motor I9. Thelatter is energizedl tover .conductors 20 from the A.C. `power line,input terminals 22. The .nlm then proceedsov'erfan additional idlerro-ller 24 and is Wound up on the take-up reel 26.

Fig'. '1 alsofshows `a television pickup or camera-tube l{28,1 which maybe of the type known as the Image Orthiconff- A tube of this generalnature, set forth in 'fanarticl 4entitled The Irn'age3'rthji'cenfle.'Sensitive Vv'- Iel'evisim Pickup 'Tubeg'byjAlb'ertjRose, `Paul K; "Weiymn' and IfIarofld B. Law.,` #whichlappeared ioni 'pages 2124-432, inclusive, 1of the Julyl 1946 lissue 'oflthe "Proc'eedings 'of kthe Institute "of" Radio '-Engiy neersifHowever; other type tubesf'having n Qa'mera tube 28`I Lincludes f'av`"lifg ,`Fl"l-t'-rc'esponsive electrode, or photo-cathodef, whichispreferably of the .laterally conducting semi-transparent character'fset 'fort-hin the Aalcovementi cned Rose etaipjarticle.-PnQte-cathciesois connect- 1 In accordance ,With "theipres'ientirrvention, potentials of alternate positive and vnega-.tivepolarconductora?. Y suchgpelranon wm '1ecer--b.eo scr'ibed in detail,4but 'for the presentjit "une assumed that the,photocathode"3!!v is at`a 'nega-- 'ti-ve ,potential with respect to i ground. Under thiscondition, rillun'iinationfa'lllig fon,V the-'translucent photo-cathode'3'0f vcauses an emission off *electrons cathode '30. ,'llhismosaicelement 3.6 is preferably composed .cfa .thin sheet of`.low-resistivity.glass .but A it ..may,.if desired, be ,constructed .of.sem-i',- conducting material as-.set .forth inthe U.. S.,Rat vent No.2,403,239 .issued toA. Rose en ,.iuiy 2, 19.46, serial4 no., 407.132,inedv August ie; 194i, .and/.0r theII. Patent 2,506,.741.1Ss1led.t0 A.V.Rose .0n

1945. 4.A suitable,electron .lens not .sl-iovvn) -vvhiclf; .mayy he` ofthe character. disclosed,.for linstancefin n mentionetliRose et al.Lar'ticlefis .employedito focus .onthe..ni0saic electrode 36 the-.e1ectrons .emitted from the surface .of photo-.cathode .30.;

Electrons released from the vphoto-Qcatllritle.320 which .are'.movedinthegdirectiomof the mosaic .so v.as to impacgtthe mosaic (36 cause, inturn,

' secondary ele'c'trons'to'be released from the .com-

l.pacted ,surface with. the number, of ysecondary electrons released byeach arriving .prima` .r,".le'lec-4 tron being governed'by numerousfactors, includ-r ingjthe velocity: of the-primary "electron at .themoment of impact, thecha'racterof the surface impacted and `s `if"'o'n'.` Such Velocity isdetermined principally bylfthe acceleratingpotentials applied to the photo-electrons, and these-potentials arepreferablyfchosen sojthat moreuthanfone secondary electron isreleasedfrom the surface ofthe mosaic electrode `36 for' each" impactingvprimary electron, o`r,-i n` other words, the sec- "ondary emissionratio'is greater than unity;

Y 'u The secondaryf electrons l'released f from the mosaic, or target,relectrode 36 are collected 'by a wire mesh screen 538:1 Thisscreen-38"'fis' normally operated atia'relatively low positive poten-4tialwith respe'c'ttoground, as determinedby a: positive charge, or;`lin; lother Words, withla negative charge deficiency. The amount of'such actual deiiciency is depender'itl upon the density ofthe electronimageat that particular point.

. The positively charged mosaic' 36 is then scan-4 v-ned by'means of1an"`el'ectron beam produced by an `electron gun atlthelopposite end ofvtube 28,

` this electron gun'bein'go any suitable type which includes acatho'defiii and agrid4 4I and an accelerating anode element. It is,therefore, illustrated more or :less schematicallyl because of .detailedexisting `descriptiondn the literature. .iIt'iwill befurthernunderstoodthat an electron multiplier structure, such. aslshown, for example,

. in the mentionedgRosefet al. article, may be incorporated in thecamera tube 28 to amplify thesignal energy available. at the signalplate.

Such a multiplierstructure :has not been shown, however, in order tosimplify vthe drawing.

The camera tube 23 also includes' a focus electrode 42, whchgmay be aconducting coating on the -wall of the tube,-and a so-calleddeceleratingring, or electrode, liwhieh is positioned in the region Aof the target35.; The focus electrode.y 42 has; applied to it a potential of positivepolarity `relative to ground,.while the decelerator ring 43, on theother'hand, is` grounded' in vorder to decelerate the scanning beamelectrons to -aplproximately zero -velocity in the lregion of `themosaic electrode i. i if. 1 L

1 Camera Vtube 23 also includes means for; deflecting the electron beamcomposed of the electrons emitted from` the cathode v40` so `that thisbeam will scan the mosaic electrode 36. Such deflecting meanslincludesAa `pair of horizontal, or scanning line,frequency-deection coils 44connected/to a horizontal deflection. generator- 46. The output of thegenerator 46 is preferably a sawtooth currentV wave, such as indicatedat 48,

and which varies at horizontal, or line-scanning, frequency. Alsoprovided is ,a pair of vertical, orfield frequency beam deection coils`il lconnected to avertical deflection generator.A 52,` the ,output ofthe `latter `being-a sawtooth. current wave ,54 varying at vertical, oreld-scanningjrequency, However,V it, will be understood thatelectrostatic` Vdeiiecting means maybe employed for deiiecting thecathode ray scanning beam of the camera tube-2B instead of theelectromagnetic deflecting means illustrated and described.

Also, various combinations o1" these two means may be used, if desired.A A

As the scanning beam travels across the surface of the.mosaic electrode36, electrons from 'the adjustment of -apotentiometer 39. `,The re- Y`lease'of secondary-electrons by a particularlele- 4ment of" the' mosaic36' leaves such element lw-ith scanning' beam toward the end of tube-28.vconf taining the electron-emitting cathode 4B. Upon arriving at `thisend` of tube "28, thereturned elec- ,trons are collected by asignalplate 56 forming a part of thetube outputcircuit-ESB. rI'he signal plate56` may be of any'suitable design, such, :for example, as a circulardiscv having `a smallcen- `tral aperture throughv which the scanningbeamelectrons produced by the cathode may pass.

For projecting optical images from the motion picture film frames on tothe photo-cathode 30. of

tube 28', a` source of light 60 is provided -with whichr is associated areflector 62. Light from the source tilV is directed upon thatportion1of i the film, l2 which `is present in the lm gate i8 by means vof acondensing lens 64. They light which ispermitted to pass through thefilm is focussed. upon the light-responsive electrode, vorphoto-catho'de` 30, byimeans of the-projection of focussing-lens systems,66; f i i.

4Since the television field s'cannings ofjthe m0- saic electrode 36 areat the rate of 60 per second, electrostatic charge images 4are developedon .the mosaic also at the rate of 60 images per second during thetimeperiod between -successivetransmissions of the separate televisionelds-thatfs i between jeach successive field-scanning operation. In Fig.2a is shown a representation -ofseveral television iields. These eldsare indicated as a4 function of time, and, asrepresented inthe drawing,each field cycle occupies 1/60 second. `Of

this cycle, not over 10%, or soo second, is. nor,- mally util-ized forthe verticalreturn of the .cathoderay scanning beam. Thisreturn periodmay .coincide `in duration', as wellpas occurrence, :with

the.` time that the electrostaticchargeimage representative of anoptical image on vther-film i2 is developedon the mosaic .electrode35,501'` other words, with the photoelectricI emissiontime .ofphotofcathode. Between each such photo- -electric emission time is aperiod normallyof ,at

i leastzyog second which is `actually the useful iieldyscanning time ofthe mosaic electrode 36. x

Fig. 2b vis relatednto Fig. 2a, and shows th vmovement which may beimparted to thelmwl 2 lin order` that television elds may -be trans-`the beam neutralize thefpositively'charged mosaic elements. The beamnormally'supplies suicient electrons to makeup the'negative chargefde'-f mittedfrom a lm'op-erating at 24U frames per second;V One nlm framecycle, as-showmoccu- '.pie'sya time period of t second; the lm `beingVmoved during a portion of this cycle Awhichmaycorrespond approximatelyto`1A50fsecond. "For a more detailed description of the relationshipAVbetween' the nlm movement and thescanningoper- `ations-of thev cameratube 28,",reference'fis made toit-he above-mentioned Seeley patentjlf I3 f Since a charge image is developed on the m'osaicielectrode 36Iduring rthe `photoele'ctricomis-- sion time of. the photo-cathodef, andsincethe latter coincides in Aoccurrence with 'the retrace 'periods ofthe cathode ray scanning beam, a prop`- "erlytestablished relationshipmust exist between the motion of the lm l2 and the scanning operatio'nofthe camera tube 28. l.This is necessary-in vorder that the movement ofthe lmlZ through y the gate |-6 yoccurs during vtheinosaicscanningperiods, or, .in other words, when no charge image is being developed onthe mosaic 36. This is also illustrated Iby the graphs aand b1. loflFig. 2.

Toy .obtain this relationship'fbetween nlm movement-'and'scanningoperation, the synchronizing (or sync) signal generator 68 is provided.The operation of generator 68 is synchronized with the operation of themotor I9 by means ofl connections 161 between the ygenerator' and thevA.C. power line input terminals 2'2 whi-ch supply power t-o'the motor I9over the conductors 20, as pre-- viously described. The sync generator68 is designed to supply 'impulses at the'rate-'of60lper second whichare fed to the t0v cycle vertical, or field, deflection generator 52,the latter operating through the deflecting coils 50 -to cause thecathode ray scanning beam of tube 28 tobe 1deflected vertically at therate of 60 deflections'per second. The sync generator 68 also suppliesimpulses Iat line frequency which are'fed tothe horizontal. or line,deection generator 46, the

latter operating through the -deecting coils' 44 `to cause the cathoderay scanning beam to scan each line `oi' the mosaic electrode 36. As a"result, va movement of the nlm .I2 by the synchro- `nous motor I9 actingthro-ugh suitable gearing (where necessary) and theintermittentfmechanism l-B is'r'elated to the horizontal and verticaldeflection ofthe cathode ray scann-ing beam of the camera tube4 28, andthe pull-down' times of the film I2, tas-represented in graph b-of` Fig.2, -will occur during the mosaic scanning periods (as shown in graph aof Fig. 2) when no photoelectricv emission occurs from the photo-cathodeA 30.* The means for preventing photoelectric emis- 'sion from thephotoc-athode 30.during the scanning intervals, and for permitting suchphotoelectric emission between each scanning period, ,willnowbedescribed. f

It has been customary in television systems 'of the type in whichoptical images are projected from a moving picture film or otherrecording medium on to the light-responsive electrode offa camera tubetov employ some means in the `optical path .for intermittently cuttingoir the light from the photo-sensitive electrode during the cathode raybeam scanning periods. 'I'his means insu-allyv took the form of a discshutter such as shown, for examplein the above-mentioned Seeleyvpat-`ent,1or .in the U. S. PatentNo. 2,455,323, issued on YNovember 13, 1948to Charles L. Townsend, Serial No. 678,547, filed June 22, 1946; In sucharrange-y ments, the operation of the disc'shut'teryvas related both tothe intermittent movement of the motion picture lm and also with 'thescanning operation ofthe camera' tube. This might be of substantiallyrectangularvoltagepulses which may, for example, have a waveform such asgenerally indicated by thev reference character 14 (see Fig.` 2a). Thepulses 'T4 have relatively narrowportions 14a which 'coincide' invtimejwith theretrace periods ofthe cathode ray scanning v -beam of thecamera tube 28, and which, in one modification, vhave thel same timeydurationias theseretrace periods. The rectangular' pulses 14 also'havepositive portions, indicated in the' drawing by the yreference character14o, which` coincide in time with the 'field-scanning periods' ofvVthe'cathode ray beam of tube 28.' 'Ihe amplitude of y'thesepositive'pulses issuch as effectively to prevent electrons released byphotoelectric.einis-'l "r sion from the photo-cathode-30 from flowinglto and reaching the mosaic 36vduring the period of their application,and is controllablein any Wellf known manner, such, for example, asbymeans of a manually-actuatable.knob 15 acting to control the output ofgenerator 12. In image ytype camera tubes now generally in `use,theamplitude of 'these positive pulses mayl have a value in the orderv of+100 volts relative to ground,.for instance, (and bearing in mind thatthe decelerating electrode 43 is illustrated atground poten-l tial byway of example) lin order to completely lstop effective photoelectricvemission from` the I photo-cathode 30. y

The negative portions '14a of the pulses 14,' as above stated, occurduring retrace time, and have an amplitude suflicient so thatphotoelectric emission from the photo-cathode 30 flows yover `to andimpinges upon the mosaic 36. It has been found in practice that anegativevoltage of -600 volts is sufficient sov that an electrostaticcharge image is developed on the mosaic 36 during the accomplished, forexample, by connecting a disc shutter so that it may be driven by thesame synchronous motor which affects the movementiof i thev motionpicture film. 4

accordance with one feature, of the present invention, 'this .ldiscshutter maybe entirely elimihated;` and light may 'be permitted Itofallupon thephoto-catho'de 30 atall times during operation of thesystem, includ-ing the interv-als duri-ng v-whichthe iilm l2 is beingmoved thro-ughthe gate I6.v Such a mode of operationv is permissible ifsome means 'are provided for preventing photol'lelectric'z emission fromthe photo-'cathode 30 even though light is being receivedifon thesurface thereof. lA preferred method of thus preventing -photoele'ctricemission at desired intervals such, for example, as during the scanningperiods of the-mosaic relectrode 36, is by applying to the time thatthis negative voltage is applied to the photo-cathode 30. Since`the'electrostatic charge image is developedv on mosaic 36 during theretrace periods -of the cathode ray scanning beam, it is necessary thatthe scanning beam be cut off, or blanked,

during the development of the vcharge image, as-

otherwise partial neutralization of the Ymosaic lelements will occur insuch a manner as to cause distortion in the output of the camera tube.Accordingly, a blanking amplifier 16 is provided lwhich isalso`synchronized from the sync gen- Y. verator v(i8. This blankingamplier16 produces- Vnegative blanking pulses, which may have' a waveform suchas indicated by the reference numeral'l8, vfor application to the' grid4| of the camera tube 28 over a conductor 19 so as to bias the grid 4|negatively and out off ,the electron scanning beam during retraceperiods, or, in other words, during the periods of charge development onthe mosaic 36. Such a cutting off y:of the electron scanning beam hasthesame eiiect, however, as a utilization of all of the scanning beamelectrons to neutralize a positively charged Y mosaic. 'In other words,it is equivalent to a white" signal output from tube 28 "over conductorv58. In order that this "white signal be pre'- vented from modulating thetelevision ,transmitter so as to transmita -white signal during 92theretrace periodsofxythe cathoderay scanning beam,i the "'blanking;pulses i 1B- fare also applied over `a :conductorl :to :a video yamplier. B l which receives the? video output of camera tube 2 8.` asdevelopedzon=a.loadiresistor Bla.: These blanking zpulses 18 actrtol.outa `on' the` signal output of amplifier 48i during'tretrace@periods.4 this being accomplished inlV any; Well-known mannenlvas by a biasing` `of the .-gridor :grids of .the y,amplier tubes. ,1' rW'LL-'L'MJ f, l, l, @Alportion of they output of the syncmgenerator(58m-.is applied toy ther-'television transmitter (inot shown) over aconductor 82gso that `synchronizing impulses `may be transmitted; along.with the videooutput otthecamera tube, 28. These ,syn-y chronizingAimpulses ;aregenerally jtransmitted both at the endofgeajch;line-scanning*interval and atV the endofleach field-scanningintervalthat is, during the horizontal and vertical retrace periods ofthe cathoderay. scanning beam.

LAs previously .,broughtout, applicants invention is not restricted touse with a' motion picture projector clevicarandl such'an apparatus yhasl been shown inthe drawingmerely by way of illustration. Forexample,V it is within the scopeof the present .invention to varyeitherior both the photoelectric emissiontirne and fthe time in whichthe mosaic is, scanned by the cathode ray beam. This may be particularlydesirable in certain special applicationsefthe inventionsf For example,lif -the camera'itube 'is arrange'dltoi'vpick upa scene which has -a low`intensity of illumination and is located at a considerable distancefrom the tube, then the tube exposure time, or, in other words, the timeduring which the photocathode of the tube has applied to it a potentialof negative polarity, `may be considerably increased. This will resultin a longer duration of the electron now to the mosaic `and permit thebuilding up on the mosaic of the tube a stronger electrostatic chargeimage which would otherwise be of limited magnitude due to a shortexposure period. The increased charge image when released will in turnbe effective to produce greater signal output and have the effect ofincreasing the brightness of the image which is reproduced at thereceiver. It will, of course, be realized that suchf an expedient is notusually practicable in cases where the scene which is picked up by thecamera tube includes any appreciable motion, since the relatively longerexthe rectangular voltage wave 14 may be made` very narrow. In thisevent, the normal mosaic scanning period may be lengthened by any amountup to that by which the normal photoelectric emission time is shortened,thereby permitting a greater number of lines to be transmitted duringeach held-scanning period. In

- order not to increase appreciably the required band width, the framerepetition frequency .may be reduced. However, it will be appreciatedthat this procedure will increase the amount of flicker which ispresentin the reproduced image, even -operationwmay be particularly suitedtforgiving an instantaneousreading or indication of `some particularcharacteristic ofY a veryI fast-moving objectV or in certain industrialuses, especially where` the indicationitself is of` primaryimportance.,` and the increased amountof v Viiiclrer can basubordinatedto the principal object of obltaining thereadingor indication. .l

L It should be understood, moreover, that applicants invention is notlimitedtothelparticular time` relationships indicated by Wayqof yexamplein Fig 2, and that the invention is broadlyapplicable to any televisiontransmitting system utilizing av camera tubevhaving an image sec-` tionfon-the purpose ofpreventing photoelectric emission ybetween thephoto-cathode andthe mosaic electrode` of the tube during the timethat'` the `latter isubeing scanned `by the tube cathode rayscanningbeam. IThis eliminates-,the necessity for cutting oi n light fromthephotocathode ,duringl such retrace `periods and, consequently,` 106sawayv with: the necessitypf employing specific apparatus for such apurpose;

As applicants system employed no moving parts,`

it yfollows that` `the usual construction and Operation of such atelevisionv transmitter is simplified.

Theyaluesof .resistor 3l and its associated condenser' 34, shouldpreferably be inl the' order of 3 megohms and 1 mfd., respectively.Similarly, the values of resistor 86 and its associated condenser 88should also be preferably in the order of 3 megohms and 1 mid.,respectively. The load resistor Sia. may have any suitable value-say,for example, 25,000 ohms.

The various operating potentials for camera tube 28 are determined inaccordance with the conditions under which the system is to be operated.However, it has been found that the Y. following voltages aresufficiently approximate to serve as a guide in selecting actual values:

. Volts Potential on photocathode 30 Potential on rotator electrode 34-100 Potential on wire mesh screen 38 +1 to +5 Potential on deceleratorelectrode 43 Potential on focus electrode 42 Potential on signal plate56 +125 to +250 Potential on cathode 40 0 Bias on grid 41 0 to 25 Whilean arrangement has been described above in which photoelectric emissionfrom the photo-cathode of an image type camera tube takes place onlyduring desired intervals, it is possible to achieve the same result bypermitting photoelectric emission to occur at all times, and

l o +50 to +150 then employ means for preventing these emitted electronsfrom reaching the mosaic or target electrode during undesired intervals.This might be accomplished' for example, by placing a high positivepotential on either the rotator electrode 34 or on some other auxiliaryelectrode (not shown) in the image section of tube 28. The electronsemitted from photo-cathode 3i! would then be collected by this highlypositive electrode, and none would reach the mosaic 36. During intervalswhen it is desired to establish a positive charge pattern on mosaic 36,however, the rotator, or other auxiliary electrode, would be at negativepotential. i

It should be noted that the wire mesh scree 38 cannot be used for the`above purpose, how- 11 ever, 4since it controls the potential mosaicvelectrode 36 builds up. o I claim: "1. A- system'comprising vatelevision camera to whintne tube havingfa photo-sensitive electrode,means 1 for ysuccessively projecting images successively recorded on anintermittently moving ilmonV to said photo-sensitive electrode in saidtelevision' camera tube.- a target electrode in rsaid camera'ktubeymeans for scanning said targetvelectrode with `an-electronscanning beam', means for gen` eratnga varying voltage, means forestablishing a predetermined time relationship Ibetween theeoperation ofsaid generating means 'and the intermittent movement of said film, andmeans forfapp'ly-ing said'voltage variation to the photo- 'sensitiveelectrode of said camera tube polarized andrtimed so lthat the mostpositive portionsjof said voltage variation ooincidewith the piiods`duri-'ng which the said target electrode is bei-nig scanned by the saidelectron scanning beam alsol include the periods of intermittentmovement ofsaid film, the most positive portions ofisaid 'voltagevariation being effective during i th'e'--periodsof their application toprevent electronsfiowirig from said photo-sensitive electrode ,tdisaidftarget electrode'.

f2-;1A` te1evi'sionfsystem comprising a cameratube'jl-iavinga'phcto-cathodefrom which photoelectrons 'v are emittedunder the action of impirligifng light; means for causing thephotoclectrcnsl which are thus emitted to impact a mosaictarget-electrode in said camera tube to thereby set-up on said mosaicvtarget electrode a. seriesof Y, discrete electrostatic charges, f'saidcamera;tube

also including means `for i developingian ielectron beam and fordeiiectlngrsaid beam toiscanlsaid mosaic target electrode within.: fa:selected scan;

ning period so as .toproduce outputsignals from said tube, means'i'orproducinga voltagevax'iation during a, majorrporti'onzof'the scanningpe.- riod duringvwhichthe saidsmosaic electrode? is being scanned by thesaid electron beam, means for applying the developed voltagevariation'to said photo-cathodeI tof prevent the effective emis' sionofsaid photo-electrons therefrom during the i selected period, and meansforfscanning 3the..sai i mosaicy target electrode :duringthe'time'swhich said photo-cathode'is-mostpositive. "l

the said developed vvoltage variation 'applied-ifo n REFERENCES citronmANKsoMERs-Q The following references `'areiof record in the le cfthispatent: l I

.UNITED STATESBATENTS g w l Date 526,622 Great Britain Sept. 23,-#1940

